A stem cell is a cell that can reproduce and has the potential to change or differentiate into another type of cell. At the present time, there are 4 basic types of stem cells:                1. Embryonic Cells are generally taken from very early fertilized embryos. They can give rise to any type of cell, but unrestrained with respect to differentiation, may result in tumor formation.        2. Parthenogenetic Cells are taken from unfertilized eggs. Since the eggs are not fertilized the stem cells they produce are genetically compatible with the woman who donated the egg.        3. “Adult” Stem Cells are found in different organs, including bone marrow, adipose tissue, umbilical cord blood and fetal tissue. Like embryonic stem cells, adult stem cells can reproduce, but less so than embryonic, and unlike embryonic cells, adult stem cells generally form cells in their own tissue type, making the chance of tumor formation much less likely.        4. Induced Pluripotent Stem Cells are differentiated cells that can be stimulated to return to an undifferentiated state.        
There are many conditions of the optic nerve and of the retina of the eye that result in blindness for which there is no effective treatment to restore or in many cases, to prevent the loss of vision.